In my last blog I explained what shutter speed is and how it affects your images. In this blog I'm going to discuss the somewhat obscure topic of sensitivity.
Sensitivity is a setting on your camera that determines how sensitive the sensor or film reel is to light. It's quite a confusing topic as it goes under many names depending on what camera you have. On most stills cameras it is usually referred to as ISO which stands for the very exciting International Standards Organisation - the main governing body that standardises the sensitivity ratings for camera sensors. ISO is applied to both digital camera sensors and also film reels. For example you can get film that is rated at ISO 500, ISO 250 and so on. Depending on the range of camera you buy, the ISO range can vary dramatically as can the cleanliness of the image at certain ISO levels. For example, my primary camera, the Canon 5D MKIII has an ISO range of 100-25600 which can then be expanded to 50-102400. At an ISO rating of 6400 it performs better than my Canon 60D due to its more powerful processors.
The lower the ISO number, the less sensitive the sensor or film is to light. Higher ISOs allow you to use faster shutter speeds and smaller apertures in lower light situations however the higher the ISO the more noise or grain is visible in the image. There is a point with every camera on the market at which the grain in the image is more visible than the subject itself making shooting at such an ISO pointless. It's important to know the limitations of your camera in this regard. the base level of ISO for stills cameras tends to be ISO 100 while for digital film cameras it is ISO 850.
ISO is also used on cinematic cameras such as the Canon C300 or RED Epic. However some cinematic cameras use a different rating system EI, or Exposure Index. This is somewhat erroneous as ISO and EI can be used side by side but have become interchangeable terms. EI is derived from one or more sensitivity measurements. It is used to determine a camera's exposure in response to a light level measurment, not the sensitivity of the sensor itself. Increasing it has the exact same effects as increasing ISO. When used on a digital camera EI usually takes into account an ISO rating with a certain number of f-stops above and below 18% grey. 18% grey is the mid point between black and white. Think of black as underexposed and white as overexposed.
The last measurement of sensitivity is Gain and is used in television cameras. Gain is measured in Db and has a base level of 0Db. It tends to increase in divisions of three so typical levels are 0Db, 3Db, 6Db all the way to 18Db. Gain has the same effect on your images as ISO and EI with the added effect of decreasing background detail as the camera removes data of unexposed areas to use the saved processing power to brighten the overall image.
All three ratings sit side by side so that there is an approximate base level when comparing ratings. ISO 850 is approximate to Gain level 0Db. Both of these are approximate to EI 400.
ISO is less a creative aspect of exposing a picture and more about allowing you to achieve the f-stop and shutter speed that you want given the light levels you are shooting in.
In my next blog I will explain f-stop and aperture.